The sixth layer of the cerebral cortex has been studied by means of the Golgi method in Carnivora (dog and cat), Artiodactyla (cow and sheep), and Primates (human) brains; a basic structural uniformity being observed in all these species. Projection neurons of lamina VIa were large and medium sized pyramidal neurons (including atypical and multiapical), small pyramidal cells, and spinous multipolar neurons with long descending axons. Projection neurons of lamina VIb were medium sized pyramidal neurons and small pyramids, horizontal pyramids, inverted pyramidal cells, spinous multipolar neurons with long descending axons and large fusiform cells. Local circuit neurons of lamina VIa were Martinotti cells, basket neurons, neurogliaform cells, sparsely spinous neurons with whirled axons, spine-free multipolar cells and bipolar neurons. Local circuit neurons of lamina VIb were sparsely spinous and spine-free multipolar cells with short axons and bipolar neurons. Marked differences were observed between gyral, intermediate and fissural regions. Fusiform and bipolar neurons were vertically arranged in the former, but were tangentially orientated in intermediate and fissural regions; inverted pyramidal cells were present in the gyrus but horizontal pyramids were the respective cells in the intermediate and fissural zones. When compared with lissencephalic species, a great horizontal fibrillary system (which is vertically arranged in gyral regions) was observed in convoluted brains. Cells of origin were fusiform neurons, bipolar cells, horizontal and inverted pyramids and pyramidal neurons (the latter by means of horizontal axonal collaterals). The great development of this cortico-cortical association system in gyrencephalic species is considered to be a major step in neocortical evolution.